1. Field of the Invention
The present invention relates to a pulse-wave propagation time basis blood pressure monitor suitable for a stress test blood pressure monitor, a blood pressure monitor of the Holter type, and a blood pressure monitor used in a care room, which are used in a field where a non-restrictive, successive and noninvasive blood pressure measurement is required for a subject. More particularly, the present invention relates to a blood pressure monitor for measuring parameter values used when blood pressure is measured by using the pulse wave propagation time without greatly varying blood pressure in a stress test, for example.
2. Related art
A blood pressure monitor using a cuff has been known as a noninvasive blood pressure monitor which is capable of measuring a blood pressure in a successive and noninvasive fashion.
In this type of blood pressure monitor, the cuff must be wound on the upper part of an arm. Accordingly, the arm is restricted by the cuff wound thereon, the weight of the cuff is for the subject to bear, and the subject's sleep will be disturbed by squeezing the arm by the cuff or noise generated when the cuff is handled.
To continuously monitor blood pressure values of a subject, when the measurement interval is 5 minutes or longer, an abrupt change of a blood pressure by a shock is possibly passed unmarked.
One of the possible ways to solve this problem is to reduce the measuring interval to about one minute. In this case, the tightening of the arm by the cuff is frequently repeated, to thereby increase a load to the subject and to the blood vessels in the portion wound by the cuff. In the extreme case, inner hemorrhage may be caused in the subject.
For the noninvasive blood pressure monitor which succeeds in solving the problems of the above-mentioned blood pressure monitor, there is known a blood pressure monitor for measuring a blood pressure by making use of a pulse wave propagation velocity (pulse wave propagation time for a fixed time).
The principle of measuring a blood pressure on the basis of a pulse wave propagation velocity will be described.
The pulse wave propagation time will first be described. As shown in FIG. 9, a specific point of a pulse wave appears in the peripheral blood vessel of the finger or the ear later than in the aorta. This delay time is a pulse wave propagation time.
A pulse wave propagation velocity corresponding to a pulse wave propagation time for a fixed distance is expressed as the function of a volumetric elasticity of the vessel. When a blood pressure rises, the volumetric elasticity of the vessel increases, the wall of the vessel becomes hard, and the pulse wave propagation velocity increases.
As a consequence, a variation of the blood pressure can be obtained from the pulse wave propagation velocity.
The blood pressure monitor based on the pulse wave propagation time must be calibrated by the values of the blood pressure measured by a blood pressure measuring method which uses the cuff or other suitable means.
For the calibration, the blood pressure and the pulse wave propagation time are measured at rest and at excercise stress, for example.
Assuming that the blood pressure and the pulse wave propagation times at rest are P1 and T1, the blood pressure and the pulse wave propagation times at excercise stress are P2 and T2, and constants (parameters) proper to subjects are .alpha. and .beta., then the blood pressure P1 and P2 are expressed by EQU P1=.alpha.T1+.beta. EQU P2=.alpha.T2+.beta.
As seen from these equations, if P1, T1, P2, and T2 are determined by measurement, the parameters .alpha. and .beta. can be calculated. If these parameters are determined, a blood pressure of a subject can be obtained by merely measuring the pulse wave propagation time.
Thus, in the blood pressure monitor using the pulse wave propagation time, the calibration for determining the parameters .alpha. and .beta. is inevitably required. For the calibration, a stress test must be carried out on the subject. The test puts a strain on the subject, and much time is taken for the calibration.
Additionally, a stress test device or a blood pressure monitor exclusively used for the stress test must used, thereby increasing the device cost.